$ \left(\dfrac{81}{256}\right)^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{256}{81}\right)^{\frac{1}{2}}$ To simplify $\left(\dfrac{256}{81}\right)^{\frac{1}{2}}$ , figure out what goes in the blank: $\left(? \right)^{2}=\dfrac{256}{81}$ To simplify $\left(\dfrac{256}{81}\right)^{\frac{1}{2}}$ , figure out what goes in the blank: $\left({\dfrac{16}{9}}\right)^{2}=\dfrac{256}{81}$ so $ \left(\dfrac{256}{81}\right)^{\frac{1}{2}}=\dfrac{16}{9}$